The present invention relates to a method for the catalytic conversion of waste gases containing hydrocarbons, halogenated hydrocarbons and carbon monoxide, especially from syntheses carried out in the preparation of vinyl chloride. In a further aspect, the present invention also relates to an apparatus for carrying out the aforesaid method.
In addition to the increasing pollution of the air by SO.sub.2 and NO.sub.x, the emission of hydrocarbons and especially halogenated hydrocarbons has become a serious problem. Thus, for example, waste gases accumulate in preparation processes carried out for making vinyl chloride which contains, in part, extremely noxious substances. A maintenance of the TA air, (Technische Anleitung Luft, i.e., the West German Clean Air Regulations for the emission of waste gases in air) "Novelle" 1986, Emissionswerte [Emission values], is best achieved by means of an additional cleaning of waste gases.
Of the great number of known measures for reducing emissions of substances which pollute the air, thermal or catalytic cleaning methods are those primarily used to this end since especially high levels of cleaning effect are achieved with them.
Thus for example, in thermal afterburning, the pollutants to be eliminated are oxidized under conditions supplying energy at temperatures up to 1500.degree. C. in a combustion chamber ("VDI-Berichte" 525, Katalytische und thermische Verfahren der Abgasreinigung [Catalytic and Thermal Methods of Cleaning Waste Gases], Meeting, April 7,8, 1985, VDI-Verlag, Dusseldorf, 1985, p. 347).
The dwell time in the combustion chamber is at least 1 s (VDI loc. cit., p. 347). In accordance with economic considerations, thermal cleaning is only advantageous in the case of high concentrations of pollutants. A considerable disadvantage resides in the costs for the generation and maintenance of the conversion temperatures as well as in the fact that the heat energy released during the afterburning process can not always be utilized in an economical fashion.
Catalytic afterburning is usually performed with the aid of noble metal catalysts at approximately 300.degree.-700.degree. C. and with dwell times up to 0.3 s. It is to be preferred in the case of low concentrations of pollutants and is distinguished in comparison to the thermal method by distinctly lower operating costs (VDI loc. cit., p. 75). However, there are limitations when using the catalytic method, based on a possible reduction of activity of the catalyst due to too high thermal loading or chemical deactivation by means of catalytic poisons present in the waste gas (VDI loc. cit., p. 75).
When using the more economical catalytic cleaning of waste gas resulting from the preparation processes used for making vinyl chloride, the problem confronting the industry involved keeping the thermal loading of the catalyst low by means of chemical techniques, thus counteracting a poisoning effect as much as possible.